An LCD is a display device utilizing the electro-optic effect of changing the molecular-orientation of a liquid crystal according to an electric field and is widely used because it operates at a low voltage, uses little electric power and faciliates in designing the size of a display pattern.
As shown in FIG. 1, a common LCD comprises a pair of substrates (2, 9) made of transparent plates such as glass plates and plastic plates, transparent electrodes (3, 6) provided on the inner surface of the substrates, liquid crystal (5) interposed between the transparent electrodes (3, 6), spacers (4) which maintain the space between the two substrates (2, 9) and red, green and blue color filters (8) of proper thickness to show appropriate spectral characteristics. A light-intercepting layer between each color filter as needed and a protective layer (7) on the surface of the filter are provided polarization plates 1, 10 are disposed as outer walls for controlling light transmittance.
Backlight (11) exposed to the LCD stimulates the 10 .mu.m thick liquid crystal (5) morecules interposed between the two electrodes (3, 6) light then passes through the color filter (8) to reproduce each color.
Usually a color filter for each color pattern is formed on the surface of a substrate by photolithography. The color filter is a core part in reproducing color patterns in LCDs such as STN LCD, TFT LCD etc., by transmitting fed, green and blue lights selectively from the backlight through a TFT drive.
Many methods for manufacturing color filters are well known. U.S. Pat. No. 4,802,743 discloses an LCD which is excellent in its durability and quality of display. The LCD employs a color filter formed by photolithography of a color resin comprising a coloring material dispersed in a low temperature-curing polyamino resin containing in its molecule a photosensitive group. U.S. Pat. No. 4,837,098 discloses a color filter formed by using a coloring material which is soluble in polyimide solution and eliminating a protective layer.
FIG. 2 illustrates a spectrum of a backlight for a conventional LCD. The backlight posessess certain inherent characteristics. Therefore, it is impossible to change the whole spectrum of the LCD and only a slight peak change can be made. Among the spectrum of the backlight, light components for blue color in the 400 to 500 nm wavelength region, green color in the 520 to 560 nm wavelength region and red color in the 600 nm and above are necessary to reproduce real colors.
The role of a red color filter is to transmit the light of the wavelength region of 600 nm and above and absorb the light of the wavelength region of 600 nm and below.
The conventional red color filter has the light transmitting spectrum of 3a shown in FIG. 3A, and when the backlight, whose spectrum is shown in FIG. 2, is transmitted through the conventional red color filter, the spectrum of 4a in FIG. 4A is obtained. As shown in FIG. 4A, the transmittance of the conventional red color filter is high at the 450 to 570 nm wavelength region in the visible light portion of the spectrum and so the green component, having a main peak of 545 nm, stands out with the red component. Hence, the light color from the red color filter no longer shows pure red but orange-side red color, giving rise to the problem of being ineffective as a red color filter.
As an example, a color filter employing conventional dyes, such as red 21P(Nippon Kakaku Co., Ltd.), DFR 02-21 (Fuji Chemical Co., Ltd.) and dyes having similar characteristics, has high transmittance at the 500 to 560 nm wavelength region and the orange-side red color in the reproduced image deteriorates the image quality. As in the red color filter, when a blue color filter plays the role of high transmittance at the wavelength region of 500 nm and below, and high absorbance at the wavelength region of 500 nm and above, blue color of high purity can be reproduced.
However, the conventional blue color filter, made of such dyes as blue 5P (Nippon Kakaku Co., Ltd.), DFB( Fuji Chemical Co., Ltd.) and so on, has a spectrum showing the light transmittance as 3c shown in FIG. 3B. FIG. 4B illustrates the spectrum showing the light transmittance (4c) when the conventional blue color filter transmits a backlight. The conventional blue color filter also has high transmittance at the 500 to 600 nm wavelength region and the green component together with the blue component in the backlight stands out, making the reproduction of pure blue difficult. Therefore, the mixed colors of blue and green comes out in the region where the blue color should be reproduced and this deteriorates the color purity.